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Remember from the presentation on “Fundamentals of ASR” we learned that there are three requirements for ASR expansion to occur; these are: A sufficient quantity of reactive silica … which is provided by the aggregate A sufficient quantity of alkali … which is supplied … predominantly … by the portland cement And a supply of moisture during service If we can limit one of these requirements … we should be able to prevent damaging If li it f th i t h ld b bl t t d i ASR from happening 2
For a consideration of these three requirements we can produce a list of measures that might work Use of a non-reactive aggregate ensures that there is insufficient reactive silica available in the concrete We should really consider the second and third bullet together … Limiting the alkali content of the concrete – perhaps through the use of low alkali cement – ensures that there is insufficient alkali for damaging ASR The use of supplementary cementing materials can also be seen as a means for limiting the alkali content … in a way … although the use SCMs may not reduce the alkali content of the concrete it can reduce the availability of the alkalis for reaction … this will be discussed later The fifth bullet concerns the use of chemical admixtures – namely lithium-based compounds. This doesn’t seem to be helpful in reducing any of our three requirements for ASR … in fact lithium is an alkali. However, lithium works by changing the path of the reaction as we will see later. Note that limiting water is not on this list. Generally it is not practical to control the exposure condition of a civil-engineering structure such as a pavement or bridge. It might be our only option when dealing with an existing structure that already has ASR and therefore already contains a sufficient amount of alkali and reactive silica whi hich h can`t b be removed d – but wi ith new constructi h ion i it i is easi ier to ad dopt one of f th he measures listed here than to eliminate water. 3
The use of a non-reactive aggregate is, perhaps, the most obvious strategy. However … how sure are we that aggregate is really non-reactive. We have to put a great deal of faith in our test methods to tell us the correct answer. We looked at some of the symptoms of ASR in this structure in a previous presentation. Well the aggregate used for this structure was tested by appropriate test methods of the day – and was deemed to be non-reactive. Sixty years later the dam is 7 inches taller than when it started!!!! 4
Of course – some of our more rapid tests fail a great many aggregates that are non- reactive and there may not be a suitable aggregate that passes this test in a given geographic area – so – in the absence of data from more reliable but longer-term tests – we have to assume the aggregates are reactive and adopt preventive measures if we want to use them. In some locations most of the aggregates may really be reactive and there may be no choice but to use them with appropriate preventive measures 5
Returning to the ubiquitous Mactaquac Dam … here is a structure that will be rebuilt in less than 20 years and the owners are considering using the same reactive aggregate once again. The reason for this is one of economy … rebuilding the concrete structures will require approximately half a million cubic yards of concrete or about three quarters of a million tons of aggregate. Sufficient aggregate will be produced by the excavation of bedrock required for the new structure and – although this same rock is responsible for the existing problems – we have to find a way to use it again. i ibl f th i ti bl h t fi d t it i 6
Stanton developed the first expansion test for ASR. He produced mortar bars using different cements and various reactive aggregates and measured the expansion of these bars when exposed to moisture. He found that the expansion with a particular aggregate was strongly influenced by the alkali content of the cement. Typical expansion results after 2 years are shown for mortar bars made with a highly reactive sand from Ventura County, California. Excessive expansion and cracking of the mortar bars only occurred with cements with alkali contents in excess of 0.7 percent N-A-2-O-E. It was from this work that in 1940 – Stanton made the recommendation that damaging ASR was unlikely provided that the cement alkalis were below 0.6% sodium equivalent. This defined the classification of low-alkali cements in the United States – low- alkali cements being cements with less than or equal to 0.6% equivalent alkalis. Many jurisdictions in the U.S. still specify the use of low-alkali cement as means of preventing damaging ASR. As we will see later in the course, such a measure is NOT sufficient to guarantee that damage does not occur. 7
This slide shows the breakdown by equivalent alkali of 69 sources of Type one Portland cement from the United States – Canada – and Mexico. The equivalent alkalis can range anywhere from 0.1 percent to 1.2 percent with just under half the values being higher than 0.60% equivalent alkalis 8
There are a number of structures that have suffered from ASR despite the use of low-alkali cement 9
This is a well-known case of a pavement constructed with a cement with less than half a percent of alkalis 10
A review of the literature will show that this is not an isolated case 11
We now know that we have to control the alkali content of the concrete not JUST the alkali content of the cement … this was discussed earlier 12
There is a threshold alkali content below which expansion may not occur with a given aggregate – this threshold will vary from one aggregate to another. Currently there is no test method for determining the threshold alkali level for a given aggregate and – as discussed previously – aggregates will generally react and cause expansion and cracking at lower alkali contents in the field than are required in laboratory tests such as the concrete prism test 13
Remember this example that was used before – blocks expand in the field with less than four pounds per cubic yard of alkali – but concrete prisms from the same mix do not expand in the laboratory 14
Here are some more examples of ASR in a dam with less than 3 kg or 5 pounds of alkali -another dam with less than 2 kg or just slightly more than 3 pounds -And a pavement with less than 2 kg or 3 and a half pounds of alkali per cubic metre of concrete I C In Canada – a range of alkali f lk li limits are used li it d dependi d di ng on th th t e type of f aggregat te and d the nature of the structure and its exposure condition 15
As a consequence of this … specifications such as the Canadian spec and the more recent AASHTO (ash-toe) recommended practice base alkali limits largely on field experience – in AASHTO the actual limit varies from 3 to 5 pounds depending on the risk of asr and the nature of the structure – this will be explained later 16
SI alternative to previous slide As a consequence of this … specifications such as the Canadian spec and the more recent AASHTO (ash-toe) recommended practice base alkali limits largely on field experience – in AASHTO the actual limit varies from 1.8 to 3 kilograms depending on the risk of asr and the nature of the structure – this will be explained later 17
Ok so what about s-c-m’s (ess-see-ems) Commonly used SCMs in North America include fly ash – ground granulated (iron) blast-furnace slag – silica fume and various natural pozzolans such as calcined clay or shale – such as metakaolin … and occasionally some volcanic ash and other materials 18
Thomas Stanton recognized the possibilities of using pozzolans to control ASR in his seminal paper in 1940. In this and later work he showed that the impact of pozzolans went beyond the effect of merely diluting the cement alkalis in the mix. 19
[Not to instructor: animation used in the slide] SCMs include fly ash, slag, silica fume and natural pozzolans and almost all sources of these materials can be used to control ASR provided they are used in sufficient quantity – so the question is – “How much is enough?” – to which the answer is … “It depends!” What does it depend on? Well of course it depends on – among other things – the composition of the SCM itself. Paradoxically – the main parameter that effects the efficiency of an SCM in terms of controlling ASR is the amount or reactive silica in the SCM. Those with a lot of silica – like silica fume – will behave like the left hand curve reducing expansion to a safe level at relatively low levels of replacement – such as 10 to 15% for silica fume. Those SCMs with lower amounts of silica and more calcium – such as Class C fly ash or slag – have to be used at much higher replacement levels (maybe 50% or more) to control expansi ion – lik like th he ri igh ht h hand d curve. The alkali content of the SCM is also important – those with higher alkali contents tend to be less effective. In addition to the composition of the SCM itself – the other parameters that affect the amount of SCM you need are the reactivity of the aggregate and the amount of alkali in the system – basically as the aggregate reactivity increases or as the amount of lk f alkali li i in th he system i increases – so too does the level l of f SCM requi ired d. 20
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